(1) Field of the Invention
The invention relates to cold cathode (high field) electron emission devices particularly to their design and control.
(2) Description of the Prior Art
Cold cathode electron emission devices are based on the phenomenon of high field emission wherein electrons can be emitted into a vacuum from a room temperature source if the local electric field at the surface in question is high enough. The creation of such high local electric fields does not necessarily require the application of very high voltage, provided the emitting surface has a sufficiently small radius of curvature.
The advent of semiconductor integrated circuit technology made possible the development and mass production of arrays of cold cathode emitters of this type. In most cases, cold cathode field emission displays comprise an array of very small emitters, usually of conical shape, each of which is connected to a source of negative voltage via a cathode line. Another set of conductive lines (called control gate lines) is located a short distance above the cathode lines at an angle (usually 90.degree.) to them, intersecting with them at the locations of the conical emitters, or microtips, and connected to a source of voltage that is positive relative to the cathode line. Both the cathode and the control gate line that relate to a particular microtip must be activated before there will be sufficient voltage to cause cold cathode emission.
The electrons that are emitted by the cold cathodes accelerate past openings in the gate lines and strike an electroluminescent panel that is located some distance above the gate lines. Thus, one or more microtips serves as a sub-pixel for the total display. The number of sub-pixels that will be combined to constitute a single pixel depends on the resolution of the display and on the operating current that is to be used.
FIG. 1 is a schematic diagram of the above-described setup. High field emission source 1 is electrically connected to cathode line 2. Control gate line 3, running orthogonal to cathode line 2, is positioned above line 2, at the height of the tip, or apex, of emitter 1. An opening in line 3 is positioned so that emitter 1 is centrally located beneath it.
In general, even though the local electric field in the immediate vicinity of a microtip is in excess of 1 million volts/cm., the externally applied voltage is under a 100 volts. However, even a relatively low voltage of this order can obviously lead to catastrophic consequences, if short circuited. Consequently, a resistor needs to be placed between either the cathode lines or the control gate lines and the power supply, as ballast to limit the current in the event of a short circuit occurring somewhere within the display.
This is schematically illustrated in FIG. 2. Ballast resistor 4 has been inserted between cathode line 2 and emitter 1. In the early art, such ballast resistors were separate from and external to the individual emitters but in recent years a number of schemes have been proposed to make it possible to supply each emitter with its own separate ballast resistor. The technology of such schemes is not yet mature but steady progress is being made.
Not shown in either FIG. 1 or 2 is an anode surface located above (downstream from) the control gate line. Such an anode surface would collect the electrons emanating from the emitters. It would also be coated with a suitable phosphor so as to light up whenever it was under electron bombardment. One problem with arrangements such as those illustrated in FIGS. 1 and 2 is that the electron beam that originates at the emitter tends to spread out, because of mutual repulsion, on its way to the anode, arriving there as a relatively diffuse spot. Additionally, the current-voltage (I-V) curve tends to be non-linear, current increasing more rapidly than voltage.
A partial solution to these problems has been described by, for example, Kane et al. (U.S. Pat. No. 5,191,217 Mar. 1993). Kane's scheme is shown schematically in FIG. 3a. Focus grid 5 has been added to the basic circuit of FIG. 1. It is connected directly to cathode line 2 and is therefore always at the same electrical potential as 2. As a consequence, as the beam passes focus grid 5 it is forced to shrink to some extent, resulting in a sharper spot at the anode. Additionally some of the electrons that comprise the outermost portions of the beam are collected by the focus grid, reducing, to some extent, the excess of current arriving at the anode.
A similar approach to that of Kane et al. has been described by Epsztein (U.S. Pat. No. 5,070,282 Dec. 1991). This is schematically illustrated in FIG. 3b. The principal differences over Kane are that the ballast resistor 4 has been inserted between the control gate and the power supply and that the modulating signal 6 is applied between the cathode line 2 and the focus grid 5. In the other schemes the modulating signal was applied between the cathode line and the control gate line.
The effectiveness of these schemes is illustrated in FIGS. 4 and 5. FIG. 4 is a simulation-based plot of current vs. voltage, curve 41 being for a basic setup, such as that of FIG. 1, while curve 42 is for the modified setup described by Kane and Epsztein (as illustrated in FIGS. 3a and 3b respectively). Curves 43 and 44 are for resistors having values of 1 megohm and 10 megohms respectively, and have been included for comparison purposes. It can be seen that for both of these curves the resistance of the devices varied from about 10 megohms, at low voltages, to about 1 megohm at higher voltages, although the variation in resistance with voltage was clearly less for the modified setups.
FIG. 5 is a cross-section of the left half of an electron source such as those shown in FIGS. 3a or 3b. Conical emitter 51 is centrally located with respect to control gate 52 and focus grid 53. Regular lines in the figure, such as 54, represent equipotential surfaces while arrowed lines such as 55 represent electron trajectories. As can be seen, the beam is still diverging as it approaches the anode (not shown, but located at about 200 microns on the vertical scale of FIG. 5). Also seen, is an example of an electron (trajectory 56) striking focus gate 53 rather than going to the anode.